No, and as far as I can think, everything about the equipment and processing is identical to things I've done a million times. (The processor is a pro lab with a good reputation; I use the camera all the dang time and lately this lens has lived on it.) Nothing unusual about the rest of the roll either.Has this happened before?
I'm inclined to think it almost has to be a flare-like event that happened at exposure, but I still think the optical coincidence that produces a tight little spectrum like that, with no significant flare effects otherwise, is pretty striking.
I mean, normally when you see a rainbow effect from flare, it's hazy at the edges and accompanied by other obvious flare artifacts. Here I see some lightening to the right of the rainbow in the upper part of the image, against the dark ceiling, but it doesn't seem to cause much loss of contrast, which normally is kind of the definition of flare.
I say it's weird! :-)